The Rh Factor

My last post dealt with the ABO blood groups. There is another blood type group that we should discuss because of significant medical importance. This is the Rh system.

There are a number of Rh antigens, but for this discussion we will broadly categorize the Rh system as Rh-positive and Rh-negative. This antigen is in addition to the ABO blood groups, i.e. you can be Type A and either Rh-positive or Rh-negative.

Unlike the ABO blood groups, which is based on different antigens, the Rh system is based on the presence or absence of the Rh antigen.

If you carry the Rh antigen, you are said to be Rh-positive. If you do not carry the Rh antigen, you are said to be Rh-negative and you can produce anti-Rh antibodies.

People that are Rh-negative have not been exposed to the Rh antigen. If they are exposed they will produce Rh antibodies. These people can only receive blood without the antigen present: Rh-negative blood. On the other hand Rh-positive people can receive blood from both Rh-negative and Rh-positive people. For a review on antibody and antigens, click here.

We already know that people with Type O blood are universal donors (see the ABO blog post). Adding the Rhesus factor information we just discussed and we find out that universal donor blood is Type O negative. Similarly, people with Type AB positive blood are universal recipients.

The Rh factor can cause significant problems for an Rh-negative mom. If an Rh-negative mom is pregnant with an Rh-positive fetus, a miscarriage, birth or any procedure that will expose the mom to the fetus’ blood, will cause the mom to build an immune response to the Rh factor. In other words, the mom will now have anti-Rh antibodies and become “sensitized” to Rh-positive blood. If this same mom becomes pregnant with another Rh-positive baby, the mother’s own anti-Rh antibodies can attack the baby both in the womb and after delivery leading to hemolytic disease of the newborn.